It is estimated that by 2020 there will be some 33 million automotive vehicles sold annually with built-in wireless connectivity, generating more than 163 million terabytes of data each year via their dozens of on-board cameras and sensor technologies. When shared across a wireless network, this data can be utilized by vehicles to give them an awareness of road conditions beyond the reach of their sensors, and thus enable the driver or the vehicle itself to better plan driving maneuvers. Vehicle to Vehicle (V2V) communications is a subset of device to device (D2D) wireless technology designed to allow automobiles to “talk” to each other.
Release 12 of the Long Term Evolution (LTE) wireless communication standard has been extended to support device to device (D2D) communications features targeting both commercial and public safety applications. Some applications enabled by Rel-12 LTE include device discovery, where a device is able to sense the proximity of another device and associated applications by broadcasting and detecting discovery messages that carry device and application identities. Another application includes direct communication based on physical channels terminated directly between devices.
One potential extension for device to device communication includes support of V2x communication (Vehicle to “anything”), where “x” includes any combination of direct communication between vehicles, pedestrians and infrastructure. V2x communications may enable forward collision warning, traffic queue warning, vulnerable road user alerts, do not pass warnings, curve speed warnings, blind intersection warnings, emergency vehicle alerts, etc. For example,
V2x communication may take advantage of a network (NW) infrastructure, when available, but basic V2x connectivity should be possible even in case of lack of network coverage. Providing an LTE-based V2x interface may be economically advantageous because of LTE economies of scale and it may enable tighter integration between communications with the network infrastructure (V2I, vehicle to infrastructure) and vehicle to pedestrian (V2P) and V2V communications, as compared to using a dedicated V2x technology.
V2x communications may carry both non-safety and safety information, where each of the applications and services may be associated with specific requirements, e.g., latency, reliability, capacity, etc. European Telecommunication Standards Institute (ETSI) has defined two types of messages for road safety: Co-operative Awareness Message (CAM) and Decentralized Environmental Notification Message (DENM).
CAM: The CAM message is intended to enable vehicles, including emergency vehicles, to notify their presence and other relevant parameters in a broadcast fashion. Such messages target other vehicles, pedestrians, and infrastructure, and are handled by their applications. The CAM message also serves as active assistance to safety driving for normal traffic. The availability of a CAM message is checked every 100 ms, yielding a maximum detection latency requirement of <=100 ms for most messages. However, the latency requirement for pre-crash sensing warning is 50 ms.
DENM: The DENM message is event-triggered, such as by braking, and the availability of a DENM message is also checked every 100 ms, and the requirement of maximum latency is <=100 ms.
The package size of CAM and DENM messages varies from 100+ to 800+ bytes and the typical size is about 300 bytes. The message is supposed to be detected by all vehicles in proximity. The SAE (Society of Automotive Engineers) has also defined the Basic Safety Message (BSM) for dedicated short range communication (DSRC) with various messages sizes defined. According to the importance and urgency of the messages, the BSMs are further classified into different priorities.
Existing communication systems rely on tight requirements for frequency and timing synchronization between transmitter and receiver. This synchronization is usually provided by the network through connectivity, using appropriate signaling.
For V2x communication, synchronization may be acquired from the cellular network (i.e., as in cellular communications), from other devices, or from Global Navigation Satellite System (GNSS) (e.g., through an absolute time reference like Coordinated Universal Time (UTC)). Synchronization may be maintained for a limited time by the wireless device by use of its internal clock. It is understood that the internal clock may result in a drift that is larger than the one typically occurring when the wireless device derives synchronization from an external source. Given the nature of V2x communications, it is likely that wireless devices will lose connectivity to the external source of synchronization at some point, even if only for a short time. This loss of connectivity may be total (e.g., absence of cellular or satellite coverage) or only partial (e.g., receiving only from a few satellites). If the transmitter is not properly synchronized to the receiver, it is likely that the transmission will fail.